Online collaborative sharing of documents is typically provided by a network-based file sharing computer system that allows multiple users to access and edit the files. Generally, documents are created and edited by a specific type of native application, such as a word processor or spreadsheet application. Many such applications provide support for tracking comments or other forms of annotation made to portions of a file, such as comments on words, sentences or paragraphs in a document, or cells in a spreadsheet. However, existing systems do not provide native programmatic functions that enable users to create comments or annotations that are linked to a specific portion of an image rather than an entire image. Instead, existing systems only provide for comments linked to the entire image, leaving users to then attempt to describe the specific portion that they are referring to in the comment. This approach fails to provide sufficient granularity and control for comments on images.